Actuator having a plurality of objective lenses installed in a lens holder used with an optical pickup

ABSTRACT

An actuator used with an optical pickup includes: a base on which a holder is fixed; a lens holder in which a plurality of installation holes are formed such that a plurality of objective lenses to record and/or reproduce optical discs having different recording densities are installed at different heights; a support member, which movably supports the lens holder, having one end coupled to the lens holder and the other end coupled to the holder; and a magnetic circuit which drives the lens holder in focusing and tracking directions. The plurality of installation holes include a first installation hole in which a first objective lens for at least one type of low density optical disc is installed, and a second installation hole in which a second objective lens for a higher density optical disc than the low density optical disc is installed. When a working distance of the first objective lens for the low density optical disc installed in the first installation hole is represented as WD 1 , and a working distance of the second objective lens for the high density optical disc installed in the second installation hole is represented as WD 2 , the first and second installation holes are provided so that the first and second objective lenses are installed to satisfy the Equation:
 
WD1≧WD2,
         a separation distance between the optical disc and the second objective lens=WD 2+α     where, α=|WD 1− WD 2|× (0.1˜1.0).       

     The second installation hole is located in a more outer circumference of the optical disc than the first installation hole.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of prior U.S. patent application Ser.No. 11/056,401 filed on Feb. 14, 2005 now U.S. Pat. No. 7,458,086, whicha continuation-in-part of U.S. patent application Ser. No. 10/247,794filed on Sep. 20, 2002 now U.S. Pat. No. 6,895,593 in the United StatesPatent and Trademark Office. This application claims the benefit ofKorean Application No. 2001-75671, filed Dec. 1, 2001 in the KoreanIndustrial Property Office, the disclosure of which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an actuator used with an opticalpickup, and more particularly, to an actuator used with an opticalpickup having a plurality of objective lenses so that a plurality ofoptical discs having different recording densities can be recordedand/or reproduced.

2. Description of the Related Art

It is well-known that digital versatile discs (DVDs) are recorded and/orreproduced using light having a wavelength of 650 nm (or 635 nm) andusing an objective lens having a numeral aperture of 0.6 (0.65 in thecase of a recordable disc). In the case of DVDs having a diameter of 120mm and a track pitch of 0.74 μm, DVDs have a capacity of 4.7 or moregigabytes for a single surface. Accordingly, DVDs are not suitable for arecording medium on which high definition (HD) moving pictureinformation is to be recorded. This is the reason that a recordingcapacity of 23 or more gigabytes for a single surface is required sothat 135-minute HD moving picture information can be recorded on therecording medium.

To meet the need of a high density recording capacity, high densityoptical discs using light having a wavelength shorter than that of a redcolor, i.e., blue light, and an objective lens having a numericalaperture greater than 0.6 and having a more narrow track have beendeveloped and standardized.

In order to obtain an adequate tolerance with respect to tilt of opticaldiscs, as the numerical aperture of an objective lens increases for highdensity, the thickness of optical discs should be reduced. Consideringallowable tolerance with respect to the tilt of optical discs, the 1.2mm thickness of compact discs (CDs) is reduced to 0.6 mm for DVDs, andthere are further possibilities that the thickness of blu-ray discs(BDs) is reduced to 0.1 mm. Regarding the numerical aperture of anobjective lens, the 0.45 numerical aperture of CDs is increased to 0.6for DVDs, and there are further possibilities that the numericalaperture for BDs is 0.85. Also, considering a recording capacity, thereare good possibilities of adopting a light source such as a blue lightsource for high density discs, i.e., BD and HD DVD. Here, high densitydiscs include a BD and a high definition DVD (HD DVD). The thickness ofHD DVDs is 0.6 mm. The numerical aperture for HD DVDs is 0.65. In thedevelopment of optical discs with new specifications, compatibility withexisting optical discs is important.

However, a special technique is required to design and manufacture anobjective lens having a high numerical aperture of about 0.85 as asingle lens. Besides, it is difficult to make the working distance of anobjective lens having a high numerical aperture be as long as that of anobjective lens for DVDs.

Accordingly, to solve the working distance problem, in a compatibleoptical pickup in which optical discs having high densities can berecorded and/or reproduced, at least one objective lens used to recordand/or reproduce CDs and/or DVDs, and an objective lens for high densityrecording having a higher numerical aperture than the previous objectivelens should be separately provided.

For driving in both directions, i.e., a focusing direction and atracking direction, an actuator for an optical pickup has a magneticcircuit. The actuator maintains an interval between an optical disc andan objective lens in the focusing direction and moves the objective lensin a desired track position (center of a track). As described above,however, the above-mentioned optical pickup adopting a plurality ofoptical discs having different recording densities requires objectivelenses corresponding to the plurality of optical discs having differentrecording densities, and thus an actuator used with an optical pickuphaving a plurality of objective lenses loads a plurality of objectivelenses on a moving portion so as to move in the focusing and trackingdirections.

Referring to FIG. 1, an actuator disclosed in Japanese PatentPublication No. Hei 10-106001 includes a lens holder (bobbin: 7) atwhich two objective lenses 2 and 3 are installed, a rotation axis 1which rotatively guides the lens holder 7, and a magnetic circuit whichdrives the objective lenses 2 and 3 in focusing and tracking directions.

The two objective lenses 2 and 3 are arranged at a predetermined angle θwith respect to the center of the rotation axis 1 and rotatesimultaneously at the predetermined angle θ with respect to the rotationaxis 1.

The magnetic circuit includes focusing and tracking coils 8 and 9installed at the lens holder 7, yokes 5 a and 5 b, and a plurality ofmagnets 6. The plurality of magnets 6 are arranged at an angle greaterthan the predetermined angle of the objective lenses 2 and 3 withrespect to the rotation axis 1. The tracking coil 9 is provided tocorrespond to the magnets 6.

In a conventional actuator having the above structure as illustrated inFIG. 1, the two objective lenses 2 and 3, arranged at the predeterminedangle θ with respect to the rotation axis 1 within one lens holder 7,rotate simultaneously by the predetermined angle θ with respect to therotation axis 1 such that a plurality of optical discs having differentrecording densities and thicknesses can be recorded and/or reproduced.Detailed descriptions of the conventional actuator illustrated in FIG. 1and disclosed in Japanese Patent Publication No. Hei 10-106001 will beomitted.

Since the above conventional actuator requires a complicated magneticcircuit to control the positions of the objective lenses 2 and 3 beforeand after the objective lenses 2 and 3 rotate simultaneously by thepredetermined angle θ with respect to the rotation axis 1, and anadditional structure in which the objective lenses 2 and 3 rotatesimultaneously by the predetermined angle θ with respect to the rotationaxis 1, the entire structure of the conventional actuator iscomplicated.

Since the above conventional actuator presents an axis perturbation typedesign, sensitivity is relatively low and non-linear properties arerelatively large, the above conventional actuator is less suitable torecord and/or reproduce high density optical discs than DVDs.

Moreover, in the above conventional actuator, compared to the structureof an existing actuator in which one objective lens is installed in abobbin, the moving portion of the above conventional actuator is large,and heavier, and thus the actuator sensitivity is further degraded.

SUMMARY OF THE INVENTION

Accordingly, to an aspect of the present invention, there is provided anactuator used with an optical pickup having an improved structure, moresimple than the entire structure of a conventional actuator for anoptical pickup, in which two objective lenses are installed in one lensholder, the weight of a moving portion of the actuator is reduced andthe actuator sensitivity is improved.

According to another aspect of the present invention, there is providedan actuator for an optical pickup including: a base on which a holder isfixed; a lens holder in which a plurality of installation holes areformed such that a plurality of objective lenses to record and/orreproduce optical discs having different recording densities areinstalled at different heights; a support member, which movably supportsthe lens holder, having one end coupled to the lens holder and the otherend coupled to the holder; and a magnetic circuit which drives the lensholder in focusing and tracking directions, the plurality ofinstallation holes include a first installation hole in which a firstobjective lens for at least one type of low density optical disc isinstalled, and a second installation hole in which a second objectivelens for a higher density optical disc than the low density optical discis installed, wherein when a working distance of the first objectivelens for the low density optical disc installed in the firstinstallation hole is represented as WD1, and a working distance of thesecond objective lens for the high density optical disc installed in thesecond installation hole is represented as WD2, the first and secondinstallation holes are provided so that the first and second objectivelenses are installed to satisfy the Equation:WD1≧WD2

-   -   a separation distance with respect to the optical disc and the        second objective lens=WD2+α    -   where, α=|WD1−WD2|×(0.1˜1.0),        and the second installation hole is located in a more outer        circumference of the optical disc than the first installation        hole.

According to an aspect of the present invention, a center of the secondinstallation hole may be located in a position corresponding to aboutR=21 mm from a center of an optical disc and a center of the firstinstallation hole may be located in a position corresponding toR=14.4-18.4 mm from the center of the optical disc.

According to an aspect of the present invention, the first and secondinstallation holes may be arranged in a direction corresponding to thedirection of a radius of the optical disc.

According to an aspect of the present invention, the low density opticaldisc may be at least one selected from a CD-family optical disc and aDVD-family optical disc, and the high density optical disc has a higherdensity than the DVD-family optical disc.

According to an aspect of the present invention, when the firstobjective lens installed in the first installation hole is a lens usedfor both a CD and a DVD, a working distance of the first objective lenswith respect to a CD is represented as WD_(CD) and a working distance ofthe first objective lens with respect to a DVD is represented asWD_(DVD), the first objective lens may be installed in the firstinstallation hole so that a separation distance with respect to theoptical disc of the first objective lens satisfies the Equation, therebyreducing the entire moving distance of the actuator:WD_(DVD)≧WD_(CD)

-   -   WD_(CD)≦a separation distance with respect to the optical disc        of the first objective lens≦WD_(DV).

According to an aspect of the present invention, the first and secondobjective lenses may be installed in the first and second installationholes, respectively, so that, when the optical disc is installed and thesecond objective lens operates, contact between the first objective lensand a stack ring formed in a CD and a DVD is prevented.

According to an aspect of the present invention, the first and secondobjective lenses may be installed in the first and second installationholes, respectively, so that, when the optical disc is installed and thesecond objective lens operates, contact between the first objective lensand a stack ring formed in a CD and a DVD is prevented.

According to an aspect of the present invention, the actuator mayfurther include a liquid crystal panel correcting a spherical aberrationcaused by a difference in thickness of optical discs and being disposedbelow the second objective lens inside the lens holder in which thefirst and second objective lenses are installed.

According to an aspect of the present invention, a printed circuit board(PCB) forming a conductive line path may be attached to one side of theliquid crystal panel.

According to an aspect of the present invention, a PCB plate may beattached to sides of the second objective lens of the lens holder andconnection between the PCB forming the conductive line path and the PCBplate may be performed by soldering in the state where an end of the PCBplate is located on the PCB forming the conductive line path.

Additional aspects and/or advantages of the invention will be set forthin part in the description which follows and, in part, will be obviousfrom the description, or may be learned by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects and advantages of the invention will becomeapparent and more readily appreciated from the following description ofthe embodiments, taken in conjunction with the accompanying drawings ofwhich:

FIG. 1 is a schematic view of a conventional actuator;

FIG. 2 is a schematic perspective view of the entire structure of anactuator used with an optical pickup according to an embodiment of thepresent invention;

FIG. 3 is a top view of the actuator of FIG. 2;

FIG. 4 is a perspective view of the lens holder illustrated in FIG. 2;

FIG. 5 is a partial cross-sectional view taken along line V-V of FIG. 4;

FIG. 6 illustrates a separation distance between an optical disc and twoobjective lenses when the two objective lenses having different workingdistances are installed in an existing actuator for an optical pickup;

FIG. 7 illustrates a separation distance between an optical disc and twoobjective lenses when the two objective lenses having different workingdistances are installed in the lens holder of the actuator used with anoptical pickup according to an embodiment of the present invention;

FIG. 8 is a schematic perspective view of a first magnetic circuitadopted in the actuator used with an optical pickup of FIG. 2;

FIGS. 9A and 9B illustrate a principle of driving the lens holder in afocusing direction by the first magnetic circuit of FIG. 8;

FIG. 10 is a schematic perspective view of a second magnetic circuitadopted in the actuator used with an optical pickup of FIG. 2;

FIGS. 11A and 11B illustrate a principle of driving the lens holder in afocusing direction by the magnetic circuit of FIG. 10;

FIG. 12 is a schematic perspective view of another embodiment of thesecond magnetic circuit of FIG. 10 adopted in the actuator used with anoptical pickup of FIG. 2;

FIG. 13 is a perspective view of a focusing magnet and internal andexternal yokes to guide a magnetic flux generated by the focusing magnetillustrated in FIG. 2;

FIG. 14 is a schematic perspective view of an actuator used with anoptical pickup according to another embodiment of the present invention;

FIG. 15 is a perspective view of a moving portion of FIG. 14;

FIG. 16 is a perspective view of FIG. 15 rotated at an angle of about180°;

FIG. 17 is a rear perspective view of FIG. 16;

FIG. 18 schematically illustrates the arrangement of first and secondobjective lenses installed in a lens holder of the actuator used with anoptical pickup of FIG. 14 and the arrangement of a liquid crystal panelfor aberration correction;

FIG. 19A is a schematic plane view of the liquid crystal panel of FIG.18;

FIG. 19B is a schematic side view of the liquid crystal panel of FIG.19A;

FIG. 19C illustrates the state in which the liquid crystal panel isinstalled in the lens holder so that it is disposed below the secondinstallation hole in which the second objective lens for a high densityoptical disc is installed;

FIG. 20A is a front cross-sectional view of the lens holder illustratinga soldering point between a PCB for forming a conductive line path and aPCB plate;

FIG. 20B is a right side view of FIG. 20A; and

FIG. 21 schematically illustrates the structure of an optical recordingand/or reproducing apparatus in which the actuator used with an opticalpickup according to the present invention is adopted.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to like elementsthroughout. The embodiments are described below in order to explain thepresent invention by referring to the figures.

An actuator used with an optical pickup according to an embodiment ofthe present invention is a two-axis driving apparatus in which aplurality of objective lenses having different working distances areinstalled in one lens holder, and a moving portion of the actuator movesindependently in focusing and tracking directions. The actuator usedwith an optical pickup according to an embodiment of the presentinvention can be adopted in optical pickups to record and/or reproducetwo kinds of optical discs, such as high density disc and DVD, havingdifferent recording densities, or in optical pickups to record and/orreproduce three or more kinds of optical discs, such as high densitydisc, DVD, and CD, having different recording densities.

FIG. 2 is a schematic perspective view of the entire structure of anactuator for an optical pickup according to an embodiment of the presentinvention, FIG. 3 is a top view of FIG. 2, FIG. 4 is a perspective viewof the lens holder illustrated in FIG. 2, and FIG. 5 is across-sectional view taken along line V-V of FIG. 4.

Referring to FIGS. 2 through 5, the actuator for an optical pickupaccording to an embodiment of the present invention includes a base 10having a holder 11 installed at one side of the base 10, a lens holder20 on which a plurality of installation holes 21 and 25 are formed sothat a plurality of objective lenses 31 and 35 having different workingdistances are installed on the lens holder 20, a support member 13(suspension), which movably supports the lens holder 20, having one endcoupled to the lens holder 20 and the other end coupled to the holder11, and a magnetic circuit which drives the lens holder 20 in focusingand tracking directions.

The plurality of objective lenses 31 and 35 includes a first objectivelens 31 used to record and/or reproduce at least one kind of low densityoptical disc having different recording densities, and a secondobjective lens 35 used to record and/or reproduce higher density opticaldiscs (hereinafter, high density optical discs) than the low densityoptical discs. The first and second objective lenses 31 and 35 havedifferent working distances. For example, the first objective lens 31can record and/or reproduce DVD-family optical discs (hereinafter,DVDs), as low density optical discs, and can additionally record and/orreproduce CD-family optical discs (hereinafter, CDs). Also, for example,the second objective lens 35 can record and/or reproduce higher densityoptical discs than DVDs. Here, the plurality of objective lenses 31 and35 may include three or more objective lenses having different workingdistances so that the plurality of objective lenses 31 and 35 can beused to record and/or reproduce three or more kinds of optical discshaving different recording densities.

The actuator according to an embodiment of the present invention isprovided so that the plurality of objective lenses 31 and 35 can beinstalled in one lens holder 20 in a direction (R direction)corresponding to the direction of a radius of an optical disc, and theactuator can be compatible with an optical pickup which requires aplurality of objective lenses. This is the reason that the opticalpickup records and/or reproduces an information signal while moving inthe direction of the radius of the optical disc in an optical discdrive.

The actuator used with an optical pickup according to an embodiment ofthe present invention is provided so that the second objective lens 35for a high density optical disc is installed in a more inner diameter ofthe optical disc than the first objective lens 31 for a low densityoptical disc. This is the reason that a region in which data recordingand/or reproduction of a high density optical disc starts in a moreinner diameter location than a region in which data recording and/orreproduction of a DVD of a low density optical disc starts.

As described above, when the actuator used with an optical pickupaccording to an embodiment of the present invention has a structure inwhich the two objective lenses 31 and 35 are installed in the lensholder 20, as illustrated in FIG. 4, a first installation hole 21 inwhich the first objective lens 31 is installed, and a secondinstallation hole 25 in which the second objective lens 35 is installedare formed in the lens holder 20. Here, the number of installation holesformed in the lens holder 20 corresponds to the number of objectivelenses to be installed.

The first and second installation holes 21 and 25 are arranged in thedirection R, as illustrated in FIGS. 4 and 5, and are provided so thatthe first and second objective lenses 31 and 35 are installed atdifferent heights with respect to each other. Specifically, aninstallation projection 21 a is formed in the first installation hole 21at a relatively deep position from an upper side opposite to the opticaldisc of the lens holder 20 such that the first objective lens 31, havinga longer working distance for a low density optical disc, can beinstalled. An installation projection 25 a is formed in the secondinstallation hole 25 at the same height as an upper side opposite to theoptical disc of the lens holder 20 (or at a position closer to the upperside of the lens holder 20 than the first installation projection 21 aformed in the first installation hole 21) such that the second objectivelens 35, having a shorter working distance for a high density opticaldisc, can be installed therein.

When a working distance of the first objective lens 31 for a low densityoptical disc installed in the first installation hole 21 is representedas WD1, and a working distance of the second objective lens 35 for ahigh density optical disc installed in the second installation hole 25is represented as WD2, the first and second installation holes 21 and 25should be provided so that the first and second objective lenses 31 and35 can be installed to satisfy the following equation:WD1≧WD2  Equation 1

-   -   a separation distance between the optical disc and the second        objective lens=WD2+α    -   where, α=|WD1−WD2|×(0.1˜1.0).

FIG. 6 illustrates a separation distance between an optical disc 50 andthe first and second objective lenses 31 and 35 when the first andsecond objective lenses 31 and 35, having different working distances,are installed in an existing actuator for an optical pickup.

FIG. 7 illustrates a separation distance between the optical disc 50 andthe first and second objective lenses 31 and 35 when the first andsecond objective lenses 31 and 35, having different working distances,are installed in the lens holder 20 of the actuator used with an opticalpickup according to an embodiment of the present invention.

As seen from the comparison of FIG. 6 and FIG. 7, if the first andsecond installation holes 21 and 25 are formed so that the first andsecond objective lenses 31 and 35 can be installed while satisfying theabove Equation 1, a basic separation distance between the secondobjective lens 35 and the optical disc 50 is WD2+α. Thus, contact(interference) between the second objective lens 35, having a shorterworking distance, and the optical disc 50 can be prevented when theoptical disc 50 is initially installed, and when the first objectivelens 31 having a longer working distance operates.

Referring back to FIGS. 2 and 3, in the actuator used with an opticalpickup according to an embodiment of the present invention, the magneticcircuit is divided into a first magnetic circuit 51 to drive the firstand second objective lenses 31 and 35 in the focusing direction, and asecond magnetic circuit 55 to drive the first and second objectivelenses 31 and 35 in the tracking direction, thereby reducing the weightof the moving portion. Here, the first and second magnetic circuits 51and 55 are installed at the same side of the lens holder 20 (preferably,but not necessarily, at a side parallel with the direction R).

Here, in an optical pickup assembly when the first and second objectivelenses 31 and 35 are installed in the lens holder 20 of the actuatoraccording to an embodiment of the present invention, the moving portionincludes the lens holder 20, the first and second objective lenses 31and 35, and a magnetic circuit portion (preferably, but not necessarily,focusing and tracking coils 53 and 57) installed in the lens holder 20.

The first magnetic circuit 51 includes the focusing coil 53 and afocusing magnet 52. The focusing coil 53 is installed at both sides ofthe lens holder 20, parallel with the direction R, and the focusingmagnet 52 is installed on the base 10 to be opposite to the focusingcoil 53.

As illustrated in FIG. 8, the focusing magnet 52 is a two-polemagnetized polarization magnet, and the focusing coil 53 is formed in arectangular shape in which long sides of the focusing coil 53 are put onthe N-pole portion 52 a and S-pole portion 52 b of the focusing magnet52. In this case, as illustrated in FIGS. 9A and 9B, portionscorresponding to a pair of long sides of the focusing coil 53 are theeffective focusing coil which contributes to generate an electromagneticforce. Likewise, when the portions corresponding to a pair of long sidesof the focusing coil 53 having a rectangular shape act as an effectivefocusing coil, the overall length of the focusing coil 53 installed inthe moving portion of the actuator can be reduced.

Here, as the N-pole portion 52 a of the focusing magnet 52 is placed onthe upper side of the S-pole portion 52 b of the focusing magnet 52 andthe magnetic field from the N-pole portion 52 a protrudes from thepaper, as illustrated in FIG. 9A, when current flows counterclockwisethrough the focusing coil 53, a force is applied in the upper directionof the portions corresponding to a pair of long sides of the focusingcoil 53, according to Flemings' left hand rule. When the current flowsclockwise through the focusing coil 53, as illustrated in FIG. 9B, theforce is applied in the lower direction of the portions corresponding toa pair of long sides of the focusing coil 53. Thus, when the polarityand amount of the current flowing through the focusing coil 53 areadjusted, the position in the focusing direction of the first and secondobjective lenses 31 and 35 installed in the lens holder 20 can becontrolled.

The second magnetic circuit 55 includes the tracking coil 57 and atracking magnet 56. In this case, the tracking coil 57 is installed atboth sides of the lens holder 20, and the tracking magnet 56 isinstalled on the base 10 to be opposite to the tracking coil 57.

As illustrated in FIG. 10, the tracking magnet 56 is a two-polemagnetized polarization magnet in the direction R, and the tracking coil57 is formed in a rectangular shape in which long sides of the trackingcoil 57 are put on the N-pole portion 56 a and S-pole portion 56 b ofthe tracking magnet 56. In this case, as illustrated in FIGS. 11A and11B, portions corresponding to a pair of long sides of the tracking coil57 are the effective tracking coil which contributes to generate anelectromagnetic force. Likewise, when the portions corresponding to apair of long sides of the tracking coil 57 having a rectangular shapeact as an effective focusing coil, the overall length of the trackingcoil 57 installed in the moving portion of the actuator can be reduced.

Here, when the N-pole portion 56 a of the tracking magnet 56 is placedon the left side of the S-pole portion 56 b of the tracking magnet 56,and the magnetic field from the N-pole portion 56 a protrudes frompaper, as illustrated in FIG. 11A, when current flows counterclockwisethrough the tracking coil 57, the force is applied in the left directionof the portions corresponding to a pair of long sides of the trackingcoil 57. When the current flows clockwise through the tracking coil 57,as illustrated in FIG. 11B, the force is applied in the right directionof the portions corresponding to a pair of long sides of the trackingcoil 57. Thus, when the polarity and amount of the current flowingthrough the tracking coil 57 are adjusted, the position in the trackingdirection of the first and second objective lenses 31 and 35 installedin the lens holder 20 can be controlled.

Alternatively, as illustrated in FIG. 12, a three-pole magnetizedpolarization magnet in the direction R may be used as the trackingmagnet 56. Two tracking coils 57 are provided so that both long sides ofthe tracking coils 57 are put on the N-pole portion 56 a and S-poleportions 56 b of the tracking magnet 56. Here, in the arrangementbetween the three-pole magnetized tracking magnet 56 and the twotracking coils 57, the direction of the force applied to the trackingcoils 57 can be determined from FIGS. 11A and 11B according to thedirection of the current flowing through the tracking coils 57, and thusdetailed descriptions thereof will be omitted.

Consequently, since the magnetic circuit can increase the length of theeffective focusing coil and the length of the effective tracking coil,the overall lengths of the focusing coil 53 and the tracking coil 57 canbe reduced, and thus the weight of the moving portion of the actuatorcan be reduced.

The actuator used with an optical pickup according to this embodiment ofthe present invention further includes an internal yoke 58 and anexternal yoke 59 so that the intensity of an effective magnetic field togenerate a driving force in the focusing direction can be increased byguiding a magnetic flux generated in the focusing magnet 52.

FIG. 13 is a perspective view of the focusing magnet 52 and internal andexternal yokes 58 and 59 to guide a magnetic flux generated in thefocusing magnet 52 illustrated in FIG. 2. The internal and externalyokes 58 and 59 may be formed monolithically of the same material asthat of the base 10. Here, one of the internal and external yokes 58 and59 may be included so as to guide the magnetic flux generated in thefocusing magnet 52.

As described above, when the actuator used with an optical pickupaccording to this embodiment of the present invention further includesthe internal and external yokes 58 and 59, the focusing magnet 52 isinstalled at one side facing the lens holder 20 of the external yoke 59,and the internal yoke 58 is located between the focusing coil 53 and thecenter of the lens holder 20. Thus, as illustrated in FIG. 4, aninsertion hole 61, into which the internal yoke 58 is inserted, isfurther included in the lens holder 20.

As illustrated in FIGS. 2 and 13, the internal yoke 58 may be used as amount for the tracking magnet 56 of the second magnetic circuit 55. Inthis case, the tracking magnet 56 is installed at a side facing thecenter of the lens holder 20 of the internal yoke 58. The tracking coil57 is installed in the lens holder 20 to be opposite to the trackingmagnet 56 in the insertion hole 61.

As described above, when the actuator used with an optical pickupaccording to this embodiment of the present invention includes theinternal and external yokes 58 and 59, and the internal yoke 58 is usedas a mount to install the tracking magnet 56, and the tracking coil 57is arranged at a side toward the center of the lens holder 20 of theinsertion hole 61, the insertion hole 61 has a size in which movement inthe focusing and tracking directions of the lens holder 20 is notaffected by the internal yoke 58, the tracking magnet 56, and thetracking coil 57 located in the insertion hole 61. Here, when the lensholder 20 largely deviates from a desired position, the lens holder 20is suspended by the internal yoke 58 inserted in the insertion hole 61,and the movement of the lens holder 20 is limited, and thus the internalyoke 58 guides the movement of the lens holder 20.

As described with reference to FIGS. 2 and 13, if the intensity of theeffective magnetic field is maximized while providing the internal andexternal yokes 58 and 59, and the focusing coil 53 is formed in arectangular shape such that both long sides of focusing coil 53 are puton the N-pole and S-pole portions 52 a and 52 b of the focusing magnet52 while providing a two-pole magnetized polarization magnet, i.e., thefocusing magnet 52, the poles being disposed in the focusing direction,the length of the focusing coil 53 to generate a magnetic driving forceof a desired size, and the corresponding occupied volume can be reduced.Also, if the tracking coil 57 is formed in a rectangular shape such thatboth long sides of the tracking coil 57 are put on the N-pole and S-poleportions 56 a and 56 b of the tracking magnet 56 while providing atwo-pole or three-pole magnetized polarization magnet, the poles beingdisposed in the direction R, the length of the tracking coil 57 togenerate a magnetic driving force of a desired size, and thecorresponding occupied volume can be reduced. Consequently, the weightof the moving portion can be reduced greatly by the structure of themagnetic circuit using the polarization magnets as the focusing andtracking magnets 52 and 56.

In the actuator used with an optical pickup according to the embodimentsof the present invention, since the two objective lenses 31 and 35 areinstalled in the lens holder 20, the weight of the objective lenses 31and 35 installed in the lens holder 20 is increased compared with anexisting actuator in which one objective lens is installed in one lensholder. However, in the actuator used with an optical pickup accordingto the embodiments of the present invention, since the magnetic circuitis divided into the first magnetic circuit 51 to move the magneticcircuit in the focusing direction and the second magnetic circuit 55 tomove the magnetic circuit in the tracking direction, and thepolarization magnets provide the focusing and tracking magnets 52 and56, the weight of the magnetic circuit (in particular, the focusing coil53 and the tracking coil 57) installed in the moving portion of theactuator can be reduced as compared with the existing actuator, andmoreover, the electromagnetic force to move in the focusing and trackingdirections is greater than that of an existing actuator. Even though themoving portion of the actuator used with an optical pickup according tothe embodiments of the present invention is heavier than that of anexisting actuator in which only one objective lens is installed in thelens holder, reduction in sensitivity can be prevented. Also, the weightof the moving portion of the actuator used with an optical pickupaccording to the embodiments of the present invention may be not heavierthan that of the existing actuator in which only one objective lens isinstalled in the lens holder.

As described above, the actuator used with an optical pickup accordingto an embodiment of the present invention is provided so that the secondobjective lens 35 for a high density optical disc is installed in a moreinner diameter of the optical disc than the first objective lens 31 fora low density optical disc. As described in a subsequent embodiment, thefirst objective lens 31 for a low density optical disc may be installedin a more inner diameter of the optical disc than the second objectivelens 35 for a high density optical disc. In this case, there areadvantages that will be described as below.

FIG. 14 is a schematic perspective view of an actuator used with anoptical pickup according to another embodiment of the present invention,FIG. 15 is a perspective view of a moving portion of FIG. 14, FIG. 16 isa perspective view of FIG. 15 rotated at an angle of about 180°, andFIG. 17 is a rear perspective view of FIG. 16. In addition, FIG. 18schematically illustrates the arrangement of first and second objectivelenses installed in a lens holder of the actuator used with an opticalpickup of FIG. 14 and the arrangement of a liquid crystal panel foraberration correction. Here, elements having substantially the same orsimilar functions as or to those of the actuator used with an opticalpickup according to an embodiment of the present invention denote thesame reference numerals as possible, and a repeated description thereofwill be omitted.

Referring to FIGS. 14 through 18, there is a difference between theactuator used with an optical pickup according to second embodiment ofthe present invention and the actuator used with an optical pickupaccording to the first embodiment of the present invention in that thefirst installation hole 21 is formed in the lens holder 20 so that thefirst installation hole is disposed in a more inner diameter of theoptical disc than the second installation hole 25 and the firstobjective lens 31 for a low density optical disc is installed in a moreinner diameter of the optical disc than the second objective lens 35 fora high density optical disc. Even in the present embodiment, the firstand second objective lenses 31 and 35 are installed in the first andsecond installation holes 21 and 25 to satisfy the above Equation 1.

Like in an embodiment of the present invention, if the second objectivelens 35 is installed in a more inner diameter of the optical disc thanthe first objective lens 31, the second objective lens 35 may beinterfered with a stack ring formed in the optical disc, for example,when a position in which an information region of the optical discstarts is accessed.

FIG. 18 illustrates the case where an optical disc 50′ having a stackring 75 is mounted on a spindle motor 70. The stack ring 75 is formed inpositions of CD and DVD, which correspond to about 16.5-19.5 mm from acenter of the optical disc 50′. The stack ring 75 protrudes from asurface of the optical disc 50′, for example, by a maximum of 0.4 mm.

When the first objective lens 31 is used as a CD/DVD and the secondobjective lens 35 is used as a BD, basic separation distances betweenthe first and second objective lenses 31 and 35 and the optical disc 50′may be 1.1 mm and 0.7 mm, respectively, as illustrated in FIG. 18. Thus,when the second objective lens 35 having a smaller basic separationdistance is disposed in an outer circumference of the optical disc 50′,collision between the second objective lens 35 and the stack ring 75 canbe prevented.

In the actuator used with an optical pickup according to the presentembodiment, the center of the second installation hole 25 may belocated, for example, in a position which corresponds 21 mm from thecenter of the optical disc 50, and the center of the first installationhole 21 may be located in a position which corresponds to 14.4-18.4 mm,for example, from the center of the optical disc 50.

As illustration in FIG. 18, when the second objective lens 35 for a highdensity optical disc is installed in a more outer diameter than thefirst objective lens 31 for a low density optical disc, even in theadoption of the optical disc 50′ having the stack ring 75, the secondobjective lens 35 having a shorter working distance may be preventedfrom colliding with the stack ring 75.

When the first objective lens 31 for a low density optical disc isinstalled in a more inner diameter than the second objective lens 35 fora high density optical disc as described above, the following advantagescan be obtained.

In other words, as shown in FIGS. 17 and 18, the actuator used with anoptical pickup according to the second embodiment of the presentinvention may further include a liquid crystal panel 80 r correcting aspherical aberration caused by a difference in thickness of opticaldiscs and being disposed below the second objective lens 35 for a highdensity optical disc of the lens holder 20. By installing the secondobjective lens 35 in an outer circumference of the optical disc 50′, aspace required for forming a conductive line of the liquid crystal panel80 can be obtained.

FIG. 19A is a schematic plane view of the liquid crystal panel 80, FIG.19B is a schematic side view of the liquid crystal panel 80, and FIG.19C illustrates the state in which the liquid crystal panel 80 isinstalled in the lens holder 20 so that it is disposed below the secondinstallation hole 25 in which the second objective lens 35 for a highdensity optical disc is installed.

Referring to FIG. 19A through 19C, the liquid crystal panel 80 mayinclude a liquid crystal portion 81 in which liquid crystal iscontained, a conductive line-forming portion 83, and a printed circuitboard (PCB) 85 for forming a conductive line path. The liquid crystalportion 81 of the liquid crystal panel 80 has a square shape, theconductive line-forming portion 83 has a rectangular shape, and a glasssubstrate is used in forming the liquid crystal portion 81 and theconductive line-forming portion 83. The PCB 85 for forming theconductive line path is attached to one side of the conductiveline-forming portion 83.

A PCB plate 87 electrically connected to the liquid crystal panel 80 isattached to sides of the second objective lens 35 of the lens holder 20,as illustrated in FIGS. 16 and 17. A connection between the PCB 85 andthe PCB plate 87 is created by soldering a protruding portion of the PCB85 and an end of the PCB plate 87 located about perpendicular to theprotruding portion of the PCB 85, as illustrated in FIGS. 20A and 20B.As such, a conductive line on the PCB 85 for forming the conductive linepath and a conductive line on the PCB plate 87 is electricallyconnected. Reference numeral 89 of FIGS. 20A and 20B denotes a solderingpoint.

FIG. 20A is a front cross-sectional view of the lens holder 20illustrating the soldering point 89 between the PCB 85 for forming aconductive line path and the PCB plate 87. FIG. 20B is a right side viewof FIG. 20A.

As in the actuator used with an optical pickup according to the firstembodiment, when the second objective lens 35 for a high density opticaldisc is installed in an outer circumference of the optical disc, asufficient space required for connecting the liquid crystal panel 80installed below the second objective lens 35 and the PCB plate 87 toeach other can be obtained.

Here, since a separation space between the lens holder 20 of theactuator used with an optical pickup and the spindle motor 70 becomesvery narrow, when the optical pickup is disposed in an innermostcircumference of the optical disc, as in FIGS. 20A and 20B, it isdifficult to obtain space for soldering a connection with a structure inwhich the PCB 85 for forming the conductive line path protrudes and thePCB plate 87 is located on the PCB 85. Thus, in this case, solderingbetween the PCB 85 and the PCB plate 87 may also be performed from theside of the lens holder 20 by bending the PCB 85 for forming theconductive line path. In this case, there is a difficulty in which ashort may occur in a bending portion.

However, as illustrated in FIGS. 20A and 20B, when the second objectivelens 35 for a high density optical disc is installed in a more outercircumference than the first objective lens 31 for a low density opticaldisc, a sufficient space can be obtained and a conductive lineconnection of the liquid crystal panel 80 can be performed in aperpendicular shape. Thus, a short does not occur in a bending portion.

As illustrated in FIG. 19C, a rectangular groove 20 a for attaching theliquid crystal panel 80 may be formed in a lower end of the lens holder20. In this case, the width of the rectangular groove 20 a may be largerthan the width of the liquid crystal panel 80, by a minimum of 0.1 mm,for example, so that the position of the liquid crystal panel 80 can beadjusted when it is attached to the lower end of the lens holder 20.

Referring to FIGS. 14 and 15, a PCB plate 88 for forming a conductiveline path of coils of a magnetic circuit, that is, the first and secondmagnetic circuits 51 and 55 may be attached to an inner circumferenceside 20 b of the optical disc which is an opposite side of a side atwhich the PCB 87 for the liquid crystal panel 80 of the lens holder 20is installed. In this case, the side 20 b may have a double steppedstructure formed as both a round shape and a planar shape, so that alaser light passing through the first installation hole 21 is notinterfered, as illustrated in FIGS. 14 and 15.

In the actuator used with an optical pickup according to the secondembodiment of the present invention, the magnetic circuit, like in FIGS.2 and 3, is divided into a first magnetic circuit 51 to drive the firstand second objective lenses 31 and 35 in the focusing direction, and asecond magnetic circuit 55 to drive the first and second objectivelenses 31 and 35 in the tracking direction. Except that a pair offocusing coils 53 are installed in a direction R at both sides of thelens holder 20 parallel with the direction R, the remaining structure issubstantially the same as that of FIGS. 2 and 3.

When a tilt driving current is applied so that a tilt driving force isapplied in the upper direction of one focusing coil of the pair offocusing coils installed at each of sides of the lens holder 20 a tiltdriving force is applied in the lower direction of another focusingcoil, radial tilt can be controlled.

Thus, when the pair of focusing coils 53 are installed at both sides ofthe lens holder 20 parallel with the direction R as described above,three-axis driving in which focusing driving, tracking driving, andradial tilt control are performed can be implemented.

In the actuator used with an optical pickup according to embodiments ofthe present invention, when the first objective lens 31 installed in thefirst installation hole 21 is a lens used for both a CD and a DVD, aworking distance WD of the first objective lens 31 with respect to a CDis represented as WD_(CD) and a working distance of the first objectivelens 31 with respect to a DVD is represented as WD_(DVD), the firstobjective lens 31 should be installed in the first installation hole 21so that a separation distance with respect to the optical disc of thefirst objective lens 31 satisfies the following equation, therebyreducing the entire moving distance of the actuator:WD_(DVD)≧WD_(CD)  Equation 2

-   -   WD_(CD)≦the separation distance between the optical disc and the        first objective lens≦WD_(DVD)

When the first objective lens 31 installed in the first installationhole 21 is a lens used for both a CD and a DVD, a separation distanceWD1 with respect to the optical disc of the first objective lens 31satisfying Equation 1 is a value between a working distance of the firstobjective lens 31 with respect to the CD and a working distance of thefirst objective lens 31 with respect to the DVD, as shown in Equation 2.In this case, when a CD or a DVD is used as the optical disc, the entiremoving distance of the actuator can be reduced.

Here, even in the actuator used with an optical pickup shown in FIGS. 2and 3, the first installation hole 21 may be formed to satisfy Equation2 and the first objective lens 31 may be installed in the firstinstallation hole 21.

Although the actuator used with an optical pickup according to theembodiments of the present invention is described and illustrated as astructure in which a lens for a CD and/or a DVD is provided for thefirst objective lens 31 and a lens for a BD is provided for the secondobjective lens 35, this is only an example, and the present invention isnot limited thereto.

That is, when a working distance of the first objective lens 31 for alow density optical disc installed in the first installation hole 21 isrepresented as WD1 and a working distance of the second objective lens35 for a high density optical disc installed in the second installationhole 25 is represented as WD2, the first and second installation holes21 and 25 may be provided so that the first and second objective lenses31 and 35 satisfy the following equation.if WD1≧WD2 (in this case, the size of WD1 and WD2 is equal to or greaterthan a minimum of 0.6 mm),  Equation 3

-   -   the separation distance between the optical disc and the first        objective lens=WD1    -   the separation distance between the optical disc and the second        objective lens=WD2+β    -   if WD1≦WD2 (in this case, the size of WD1 and WD2 is equal to or        greater than a minimum of 0.6 mm),    -   the separation distance between the optical disc and the first        objective lens=WD1+β    -   the separation distance between the optical disc and the second        objective lens=WD2    -   where, β≧0.1.

When the first and second installation holes 21 and 25 are provided sothat the first and second objective lenses 31 and 35 are installed tosatisfy Equation 3, if the optical disc is installed and the firstobjective lens 31 or the second objective lens 35 operates, contactbetween the optical disc and the first objective lens 31 or the secondobjective lens 35 can be prevented.

When the above equation is satisfied, an optical recording and/orreproducing apparatus having the actuator used with an optical pickupaccording to the first embodiment or the second embodiment of thepresent invention may adopt a CD and/or a DVD for a low density opticaldisc and an HD DVD for a high density optical disc, for example.

Although the actuator used with an optical pickup according to theembodiments of the present invention is described and illustrated as astructure in which the two objective lenses 31 and 35 having differentworking distances are installed in one lens holder 20, this is only anexample, and the present invention is not limited thereto. That is, theactuator used with an optical pickup according to the embodiments of thepresent invention has a structure in which two or more objective lensesare installed in one lens holder, and thus can be adopted in the opticalpickup such that two or three or more kinds of optical discs havingdifferent recording densities, such as CDs, DVDs, and high densityoptical discs can be compatible with and recorded and/or reproduced.

The optical pickup in which the above-mentioned actuator according tothe embodiments of the present invention can be adopted may have astructure in which an optical system to record and/or reproduce highdensity optical discs and an optical system to record and/or reproducelow density optical discs, such as DVDs (and additionally, CDs), areseparately included. Specific examples of an optical structure of theoptical pickup adopting the actuator according to the embodiments of thepresent invention can be understood by people skilled in the art, andthus detailed descriptions and illustrations of specific examples of theoptical structure of the optical pickup adopting the actuator accordingto the embodiments of the present invention will be omitted.

Hereinafter, the operation of an optical disc drive in which theactuator used with an optical pickup according to an embodiment of thepresent invention is adopted will be described.

If the optical disc is inserted into the optical disc drive, the type ofoptical disc is determined by a photodetector installed in the opticalpickup or a separate detection device. The type of optical discs isdiscriminated according to a recording density.

In the optical disc drive in which the actuator used with an opticalpickup according to an embodiment of the present invention is adopted,when the discrimination of the type of the optical disc is completed,focusing and tracking servos for an objective lens corresponding to thetype of the optical disc sequentially operate. That is, if the insertedoptical disc is a high density optical disc, the focusing and trackingservos operate such that the objective lens, arranged in an innerdiameter of the lens holder, is located in a proper position of theoptical disc. Also, if the inserted optical disc is a low densityoptical disc (for example, a DVD or CD), the focusing and trackingservos operate such that the objective lens, arranged in an outerdiameter of the lens holder, is located in a proper position of theoptical disc.

In the optical disc drive in which the actuator used with an opticalpickup according to the second embodiment of the present invention isadopted, when the discrimination of the type of the optical disc iscompleted, focusing and tracking servos for an objective lenscorresponding to the type of the optical disc sequentially operate. Thatis, if the inserted optical disc is a high density optical disc, thefocusing and tracking servos operate such that the objective lens,arranged in an outer diameter of the lens holder, is located in a properposition of the optical disc. Also, if the inserted optical disc is alow density optical disc (for example, a DVD or CD), the focusing andtracking servos operate such that the objective lens, arranged in aninner diameter of the lens holder, is located in a proper position ofthe optical disc.

Here, the focusing and tracking servos refer to focusing and trackingerror signals detected by the photodetector installed in the opticalpickup, allowing current to flow through the focusing and tracking coilsattached to the moving portion of the actuator, thereby generating thedisplacement of the moving portion. When the current flows through thefocusing and tracking coils, an electromagnetic force is generated bythe interaction between the current flowing through the focusing andtracking coils and the magnetic flux generated in the focusing magnetand the tracking magnet such that the lens holder is moved in thefocusing and tracking directions.

FIG. 21 schematically illustrates the structure of an optical recordingand/or reproducing apparatus in which the actuator used with an opticalpickup according to the present invention is adopted.

Referring to FIG. 21, the optical recording and/or reproducing apparatusincludes a spindle motor 312 for rotating an optical disc 100, anoptical pickup 300 being installed to move in the direction of a radiusof the optical disc 100 and reproducing information recorded on theoptical disc 100 or recording information on the optical disc 100, adriving unit 307 for driving the spindle motor 312 and the opticalpickup 300, and a control unit 309 for controlling focusing, trackingand/or tilt servos of the optical pickup 300. Here, reference numeral352 denotes a turntable and reference numeral 353 denotes a clamp forchucking the optical disc 100.

The optical pickup 300 includes an optical pickup optical system havingthe first and second objective lenses 25 and 35, arranged in a directionR, for focusing light emitted from a light source onto the optical disc100, and an optical pickup actuator for driving the first and secondobjective lenses 25 and 35. In this case, the actuator used with anoptical pickup according to one of the embodiments of the presentinvention as described previously can be adopted for the optical pickupactuator.

The light reflected from the optical disc 100 is detected by aphotodetector installed in the optical pickup 300, photoelectricallytransformed, and is changed into an electrical signal, and theelectrical signal is inputted into the control unit 309 through thedriving unit 307. The driving unit 307 controls a rotational speed ofthe spindle motor 312, amplifies the inputted signal, and drives theoptical pickup 300. The control unit 309 allows the optical pickup 300to operate a focusing, a tracking and/or a tilt operation bytransmitting a focusing servo, a tracking servo and/or a tilt servocommand controlled based on the signal inputted from the driving unit307, to the driving unit 307.

As described above, the actuator used with an optical pickup accordingto the embodiments of the present invention has a structure in which twoobjective lenses are installed in one lens holder, and a device requiredto rotate the objective lenses simultaneously by a predetermined anglewith respect to the rotation axis, unlike with a conventional actuatorto present an axis perturbation type, is not required, and thus theentire structure of the actuator is simplified.

Also, the actuator used with an optical pickup according to theembodiments of the present invention has a structure in which a magneticcircuit to adjust the lens holder in the tracking direction, and amagnetic circuit to adjust the lens holder in the focusing direction aredivided, and thus the weight of the moving portion of the actuator canbe reduced, and high sensitivity can be achieved.

Further, the objective lens for a high density optical disc, having ashorter working distance, is installed in the lens holder satisfyingEquation 1 supra, and thus interference between the objective lens for ahigh density disc and the optical disc can be prevented when the highdensity optical disc is initially installed or a low density opticaldisc is installed.

Furthermore, in the actuator used with an optical pickup according toanother embodiment of the present invention, collision between theoptical disc having the stack ring and the objective lens for a highdensity can be prevented.

Furthermore, the actuator used with an optical pickup according to theembodiments of the present invention has a structure in which theobjective lens, having a longer working distance, is installed at adeeper position from the upper side of the lens holder than theobjective lens having a shorter working distance, and thus cancontribute to make the optical pickup thinner.

Although a few embodiments of the present invention have been shown anddescribed, it would be appreciated by those skilled in the art thatchanges may be made in this embodiment without departing from theprinciples and spirit of the invention, the scope of which is defined inthe claims and their equivalents.

1. An actuator used with an optical pickup, the actuator comprising: alens holder in which a plurality of installation holes is formed suchthat a plurality of objective lenses, to record and/or reproduce opticaldiscs having different recording densities, is installed at differentheights; and a support member which movably supports the lens holder,wherein the plurality of installation holes includes: a firstinstallation hole in which a first objective lens for a first opticaldisc is installed, and a second installation hole in which a secondobjective lens for a second optical disc is installed, wherein thesecond optical disc has a higher recording density than the firstoptical disc, wherein, when the first objective lens is used to recordand/or reproduce the first optical disc, the lens holder is positionedsuch that a working distance from the first optical disc to the firstobjective lens is represented as WD1, wherein, when the second objectivelens is used to record and/or reproduce the second optical disc, thelens holder is positioned such that a working distance from the secondoptical disc to the second objective lens is represented as WD2, whereinWD1≧WD2, wherein, when the first objective lens is used to record and/orreproduce the first optical disc, the lens holder is positioned suchthat a separation distance from the first optical disc to the secondobjective lens equals WD2 plus alpha, and wherein, if each respectiveworking distance WD1 and WD2 is equal to or greater than a minimum of0.6 mm, then alpha is equal to or greater than 0.1 mm.
 2. A method,comprising: movably supporting a lens holder having a first installationhole and a second installation hole formed therein; providing a firstobjective lens for recording and/or reproducing a first optical disc inthe first installation hole of the lens holder; providing a secondobjective lens for recording and/or reproducing a second optical disc inthe second installation hole of the lens holder, the second optical dischaving a higher recording density than the first optical disc;positioning the lens holder such that a working distance from the firstoptical disc to the first objective lens is represented as WD1 when thefirst objective lens is used to record and/or reproduce the firstoptical disc; positioning the lens holder such that a working distancefrom the second optical disc to the second objective lens is representedas WD2 when the second objective lens is used to record and/or reproducethe second optical disc; and positioning the lens holder such that aseparation distance from the first optical disc to the second objectivelens equals WD2 plus alpha, when the first objective lens is used torecord and/or reproduce the first optical disc, wherein WD1≧WD2, andwherein, if each respective working distance WD1 and WD2 is equal to orgreater than a minimum of 0.6 mm, then alpha is equal to or greater than0.1 mm.